<h2>DESCRIPTION</h2>

<!-- TODO: the use of Addon and extension is not coherent -->

<em>g.extension</em> downloads and installs, removes or updates
extensions (addons) from the official
<a href="https://grass.osgeo.org/grass8/manuals/addons/">GRASS GIS Addons repository</a>
or from user-specified source code repositories into the local GRASS GIS
installation.
<p>
Two types of extensions are supported:
<ul>
<li>Python scripts: they are installed without the need of compilation or (usually)
    the need of special dependencies.</li>
<li>Source code (mostly written in C programming language; may also be written
    in C++, Fortran or other languages): while on MS-Windows systems the requested
    GRASS GIS extension is downloaded pre-compiled from the GRASS GIS site, on Unix
    based systems the installation is preceded by the automated download of the
    extension's source code along with subsequent compilation and installation.
    This requires a compiler environment to be present on the user's computer.</li>
</ul>

<h3>Managing installed extensions</h3>

<p>Re-running <em>g.extension</em> on an installed GRASS GIS Addon
extension re-installs the requested extension which may include
updates.
<p>
To bulk-update all locally installed GRASS GIS extensions, 
<em><a href="g.extension.all.html">g.extension.all</a></em> module
is available.


<h3>Where the extensions are installed</h3>

GRASS GIS extensions are installed by <em>g.extension</em> into a dedicated
directory.
The default is a directory for application data and settings inside
the user's home directory. 
On GNU/Linux it is <tt>$HOME/.grass8/addons</tt>,
on MS-Windows it is <tt>%APPDATA%\Roaming\GRASS8\addons</tt>.
The name of the directory is stored in the <tt>GRASS_ADDON_BASE</tt>
environmental variable.

<p>
The flag <b>-s</b> changes this install target directory to the GRASS GIS
installation directory
(determined by <tt>GISBASE</tt> environmental variable, e.g. <tt>/usr/</tt>)
rather than the default directory defined as per  <tt>GRASS_ADDON_BASE</tt> 
(see also documentation for <a href="variables.html">variables</a>).
<em>g.extension</em> checks if the user has permission to write to
<tt>GISBASE</tt> or <tt>GRASS_ADDON_BASE</tt>.

<p>
The place where the extensions are installed can be customized by
the option <b>prefix</b>. Ensuring that these extensions will be accessible
in GRASS GIS is in this case in the responsibility of the user.


<h3>Source code sources and repositories</h3>

<h4>GRASS GIS Addons repository on GitHub</h4>
By default, <em>g.extension</em> installs extensions from the official
GRASS GIS Addons GitHub repository. However, different sources can be specified
using the <b>url</b> option.

<p>
Individual extensions can also be installed by providing a URL to the
source code on GitHub or OSGeo Trac. The latter, however, works only for certain
directories where the download of ZIP files was enabled by project administrators
of the trac server.

<h4>Local source code directory</h4>
Optionally, new extension can be also installed from a source code placed
in a local directory on disk. This is advantageous when developing
a new module.
To keep the directory clean, the directory content is copied
to a temporary directory and the compilation happens there.

<h4>Local source code ZIP file</h4>
In addition, new extension can be also installed from a ZIP file
or an archive file from the TAR family (e.g., <tt>.tar.gz</tt> or <tt>.bz2</tt>).
The file can be on disk (specified with a path), or on web (specified by
an URL).

<h4>Online repositories: GitHub, GitLab and Bitbucket</h4>
For well known general hosting services, namely GitHub, GitLab and Bitbucket,
<em>g.extension</em> supports the download of a repository as a ZIP file.
Here the user only needs to provide a base URL to the repository web page
(with or without the <tt>https://</tt> part).
For GitLab and Bitbucket, the latest source code in the default branch is 
downloaded, for GitHub, the latest source code in the master branch is downloaded.
Of course, a user can still specify the full URL of a ZIP file
and install a specific branch or release in this way (ZIP file mechanism
will be applied).

<p>
For the official repository, <em>g.extension</em> supports listing available
extensions (addons) and few other metadata-related operations which
depend on a specific infrastructure.
For other sources and repositories, this is not supported because it is
assumed that other sources contain only one extension, typically a module
or group of modules.

<h4>Needed directory layout</h4>
When none of the above sources is identified, <em>g.extension</em> assumes
<!-- TODO: update this description -->
that the source is in a GitHub repository and uses the <em>svn</em> command
line tool to obtain the source code. The expected structure of the repository
should be the same as the one of the official repository.

<p>
Non-official sources are supported on all operating systems except for
MS-Windows.


<h3>Compilation and installation</h3>

On MS-Windows systems, where compilation tools are typically not readily
locally installed, <em>g.extension</em> downloads a precompiled executable
from the GRASS GIS project server. On all other operating systems
where it is not difficult to install compilation tools,
<em>g.extension</em> downloads the source code of the requested
extension (addon) and compiles it locally.
This applies for both C and Python modules
as well as any other extensions. The reason is that more things such
as manual page are compiled, not only the source code (which is really
necessary to compile just in case of C).


<h2>EXAMPLES</h2>

<h3>Download and install of an extension</h3>

Download and install <em>r.stream.distance</em> into current GRASS installation

<div class="code"><pre>
g.extension extension=r.stream.distance
</pre></div>

This installs the extension from the official repository.
For convenience, a shorter syntax can be used:

<div class="code"><pre>
g.extension r.stream.distance
</pre></div>

<h3>Download and install of an extension when behind a proxy</h3>

Example for an open http proxy:
<div class="code"><pre>
# syntax: http://proxyurl:proxyport
g.extension extension=r.stream.distance proxy="http=http://proxy.example.com:8080"
</pre></div>

<p>
Example for a proxy with proxy authentication:
<div class="code"><pre>
# syntax: http://username:password@proxyurl:proxyport
g.extension extension=r.stream.distance proxy="http=http://username:password@proxy.example.com:8080"
</pre></div>

<h3>Managing the extensions</h3>

List all available extensions in the official GRASS GIS Addons repository:

<div class="code"><pre>
g.extension -l
</pre></div>

List all locally installed extensions:

<div class="code"><pre>
g.extension -a
</pre></div>

Removal of a locally installed extension:

<div class="code"><pre>
g.extension extension=r.stream.distance operation=remove
</pre></div>

<h3>Installing from various online repositories: GitHub, GitLab, Bitbucket</h3>

Simple URL to GitHub, GitLab, Bitbucket repositories:

<div class="code"><pre>
g.extension r.example.plus url="https://github.com/wenzeslaus/r.example.plus"
</pre></div>

Simple URL to GitHub, GitLab, Bitbucket repositories from a specific (e.g. development) branch:

<div class="code"><pre>
g.extension r.example.plus url="https://github.com/wenzeslaus/r.example.plus" branch=master
</pre></div>

Simple URL to OSGeo Trac (downloads a ZIP file, requires download to be enabled in Trac):

<div class="code"><pre>
g.extension r.example url=trac.osgeo.org/.../r.example
</pre></div>

In general, when a ZIP file or other archive is provided, the full URL can be used:

<div class="code"><pre>
g.extension r.example url=http://example.com/.../r.example?format=zip
</pre></div>

Note that because of MS-Windows operating system architecture,
only official repository is supported on this platform.

<h3>Install a specific version from Addons</h3>

To install a specific version from GRASS GIS Addons, specify the full
URL pointing to Trac code browser and include Subversion revision
number. For example, this installs the version number 57854 of
r.local.relief module:

<div class="code"><pre>
g.extension r.local.relief url="https://trac.osgeo.org/grass/browser/grass-addons/grass7/raster/r.local.relief?rev=57854&amp;format=zip"
</pre></div>

<h3>Installing when writing a module locally</h3>

Having source code of a GRASS module in a directory on disk
one can install it using:

<div class="code"><pre>
g.extension r.example url=/local/directory/r.example/
</pre></div>


<h2>KNOWN ISSUES</h2>

Toolboxes in the official repository cannot be downloaded.
<p>
On MS-Windows, only the official repository is working
because there is no way of compiling the modules
<!-- what does it mean?? -->
(a Python replacement for Python scripts should be implemented).

<h2>TROUBLESHOOTING</h2>

Since extensions have to be compiled on Unix based systems (Linux, Mac OSX etc.)
unless a Python extension is installed, a full compiler environment must be
present on the user's computer.

<h3>ERROR: Please install GRASS development package</h3>

While GRASS GIS is available on the user's computer, the respective development
package is lacking. If GRASS was installed from a (Linux) repository, also the
grass-dev* package (commonly named "grass-dev" or "grass-devel", sometimes along
with the version number) must be installed.

<h2>SEE ALSO</h2>

<em>
<a href="g.extension.all.html">g.extension.all</a>
</em>

<p>
<a href="https://grass.osgeo.org/grass8/manuals/addons/">GRASS GIS 8 Addons Manual pages</a>
<br>
<a href="https://grasswiki.osgeo.org/wiki/GRASS_AddOns">GRASS Addons</a> wiki page.

<h2>AUTHORS</h2>

Markus Neteler (original shell script)<br>
Martin Landa, Czech Technical University in Prague, Czech Republic (Python rewrite)<br>
Vaclav Petras, <a href="http://gis.ncsu.edu/osgeorel/">NCSU OSGeoREL</a> (support for general sources, partial refactoring)

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